1. Field of the Invention
The invention pertains to a xe2x80x9cleads over chipxe2x80x9d (LOC) semiconductor die assembly and, more particularly, to a method and apparatus for reducing the stress resulting from lodging of filler particles present in plastic encapsulants between the undersides of the lead frame leads and the active surface of the die.
2. State of the Art
The use of LOC semiconductor die assemblies has become relatively common in the industry in recent years. This style or configuration of semiconductor device replaces a xe2x80x9ctraditionalxe2x80x9d lead frame with a central, integral support (commonly called a die-attach tab, paddle, or island) to which the back surface of a semiconductor die is secured, with a lead frame arrangement wherein the dedicated die-attach support is eliminated and at least some of the leads extend over the active surface of the die. The die is then adhered to the lead extensions with an adhesive dielectric layer of some sort disposed between the undersides of the lead extensions and the die. Early examples of LOC assemblies are illustrated in U.S. Pat. No. 4,862,245 to Pashby et al. and U.S. Pat. No. 4,984,059 to Kubota et al. More recent examples of the implementation of LOC technology are disclosed in U.S. Pat. Nos. 5,184,208; 5,252,853; 5,286,679; 5,304,842; and 5,461,255. In instances known to the inventors, LOC assemblies employ large quantities or horizontal cross-sectional areas of adhesive to enhance physical support of the die for handling.
Traditional lead frame die assemblies using a die-attach tab place the inner ends of the lead frame leads in close lateral proximity to the periphery of the active die surface where the bond pads are located, wire bonds then being formed between the lead ends and the bond pads. LOC die assemblies, by their extension of inner lead ends over the die, permit physical support of the die from the leads themselves as well as more diverse (including centralized) placement of the bond pads on the active surface, as well as the use of the leads for heat transfer from the die. However, use of LOC die assemblies in combination with plastic packaging of the LOC die assembly, as known in the art, has demonstrated some shortcomings of LOC technology as presently practiced in the art.
By far the most common manner of forming a plastic package about a die assembly is molding, and specifically transfer molding. In this process (and with specific reference to LOC die assemblies), a semiconductor die is suspended by its active surface from the underside of inner lead extensions of a lead frame (typically Cu or Alloy 42) by a tape, screen print or spin-on dielectric adhesive layer. The bond pads of the die and the inner lead ends of the frame are then electrically connected by wire bonds (typically Au, although Al and other metal alloy wires have also been employed) by means known in the art. The resulting LOC die assembly, which may comprise the framework of a dual-in-line package (DIP), zig-zag in-line package (ZIP), small outline j-lead package (SOJ), quad flat pack (QFP), plastic leaded chip carrier (PLCC), surface mount device (SMD) or other plastic package configuration known in the art, is placed in a mold cavity and encapsulated in a thermosetting polymer which, when heated, reacts irreversibly to form a highly cross-linked matrix no longer capable of being re-melted.
The thermosetting polymer generally is comprised of three major components: an epoxy resin, a hardener (including accelerators), and a filler material. Other additives such as flame retardants, mold release agents and colorants are also employed in relatively small amounts. While many variations of the three major components are known in the art, the focus of the present invention resides in the filler materials employed and their effects on the active die surface.
Filler materials are usually a form of fused silica, although other materials such as calcium carbonates, calcium silicates, talc, mica and clays have been employed for less rigorous applications. Powdered fused quartz is currently the primary filler used in encapsulants. Fillers provide a number of advantages in comparison to unfilled encapsulants. For example, fillers reinforce the polymer and thus provide additional package strength, enhance the thermal conductivity of the package, provide enhanced resistance to thermal shock, and greatly reduce the cost of the polymer in comparison to its unfilled state. Fillers also beneficially reduce the coefficient of thermal expansion (CTE) of the composite material by about fifty percent in comparison to the unfilled polymer, resulting in a CTE much closer to that of the silicon or gallium arsenide die. Filler materials, however, also present some recognized disadvantages, including increasing the stiffness of the plastic package, as well as the moisture permeability of the package.
One previously unrecognized disadvantage discovered by the inventors herein is damage to the active die surface resulting from encapsulant filler particles becoming lodged or wedged between the underside of the lead extensions and the active die surface during transfer molding of the plastic package about the die and the inner lead ends of the LOC die assembly. The filler particles, which may literally be jammed in position due to deleterious polymer flow patterns and flow imbalances in the mold cavity during encapsulation, place the active die surface under residual stress at the points of contact of the particles. The particles may then damage the die surface or conductive elements thereon or immediately thereunder when the package is further stressed (mechanically, thermally, electrically) during post-encapsulation handling and testing.
While it is possible to employ a lower volume of filler in the encapsulating polymer to reduce potential for filler particle lodging or wedging, a drastic reduction in filler volume raises costs of the polymer to unacceptable levels. Currently available filler technology also imposes certain limitations as to practical, beneficial reductions in particle size (currently in the 75 to 125 micron range, with the larger end of the range being easier to achieve with consistency) and in the shape of the filler particles. While it is desirable that particles be of generally spherical shape, it has thus far proven impossible to eliminate non-spherical flakes or chips which, in the wrong orientation, maximize stress on the die surface.
Ongoing advances in design and manufacturing technology provide increasingly thinner conductive, semiconductive and dielectric layers in state-of-the-art dies, and the width and pitch of conductors serving various purposes on the active surface of the die are likewise being continually reduced. The resulting die structures, while robust and reliable for their intended uses, must nonetheless become more stress-susceptible due to the minimal strength provided by the minute widths, depths and spacings of their constituent elements. The integrity of active surface die coats such as silicon dioxide, doped silicon dioxides such as phosphorous silicate glass (PSG) or borophosphorous silicate glass (BPSG), or silicon nitride, may thus be compromised by point stresses applied by filler particles, the result being unanticipated shortening of device life if not immediate, detectable damage or alteration of performance characteristics.
The aforementioned U.S. Pat. No. 4,984,059 to Kubota et al. does incidentally disclose several exemplary LOC arrangements which appear to greatly space the leads over the chip or which do not appear to provide significant areas for filler particle lodging. However, such structures may create fabrication and lead spacing and positioning difficulties.
To the inventors"" knowledge, those of ordinary skill in the art have failed to recognize this particular stress phenomenon attendant to transfer molding and the use of filled encapsulants. The current state of the art provided an LOC structure which beneficially accommodates this phenomenon.
The present invention provides the lead-supported die assembly of a conventional LOC arrangement, but substantially reduces the direct adhesion of the dies to elements of the lead frame to promote more flexibility in bending and torsion in the leads-to-die attachment mechanism to beneficially accommodate the presence of filler particles of the polymer encapsulant between the leads and the active die surface. The minimization of the pre-encapsulation direct leads-to-die adhesion permits lead flexure in response to the introduction of an underlying filler particle or particles during the transfer molding process, and thus an immediate reduction in the residual stress experienced by the active die surface responsive to the filler introduction. This lessened residual stress is carried forward in the encapsulated package after cure, permitting the package to better withstand the stresses of post-encapsulation handling and testing, including the elevated potentials and temperatures experienced during burn-in, without adverse effects.
The LOC apparatus of the present invention comprises a die and a lead frame, to which the active surface of the die is directly adhered over a minimum cross-sectional area of lead frame surface with a dielectric adhesive to permit the lead frame to physically support the die during pre-encapsulation handling and processing such as wire bonding, while providing an enhanced degree of lead flexure during the encapsulation process. With such an arrangement, intrusion of filler particles between the inner lead ends and the active surface of the die during the encapsulation process is beneficially accommodated.
Stated in more specific terms and on the scale of an individual lead and the underlying active surface of the die, the dielectric adhesive (tape, screen print or spin-on, as known in the art) disposed on the underside of a lead is minimized in both cross-sectional area and longitudinal extent along the lead axis. This design permits the free end of the lead interior of the adhesion point of the lead to the die to flex in bending and torsion, so as to bend or twist in the presence of a filler particle lodged between that lead and the die.
Characterized in yet another manner, the invention contemplates the application of a dielectric adhesive onto the lead frame in a pattern calculated to minimize the required cross-sectional area of adhesive while providing adequate physical support for the die by the lead frame.
Characterized in yet another manner, the invention contemplates the application of a dielectric adhesive to certain lead frame elements, such as buses, in a substantially continuous manner so as to adequately support the die and prevent filler particle intrusion between those elements and the die, while permitting other lead frame elements, such as inner lead ends, to substantially completely free-float and flex with respect to the die surface before and during the encapsulation process.
It is also contemplated that certain lead frame elements, such as particular leads, may be configured with enlarged inner lead ends to receive the dielectric adhesive to provide required physical support for the die, while other lead frame elements are permitted to free-float and flex during the encapsulation process.